In the conventional activated sludge method, cleaning performance is achieved in an activated sludge tank by activated sludge (bacterial sludge). In order to concentrate active biomass in the activated sludge tank, the activated sludge that is gravimetrically thickened in a secondary clarifier tank is pumped back in the activated sludge tank with a return ratio of up to 150% relative to the average amount of dry water. This results in a dry substance content TSBB of about 3 to 4 g/l in the activated sludge tank. This limits the performance of the wastewater purification plant and increases the hydraulic load on the activated sludge tank and secondary clarifier tank. Furthermore, a large amount of energy is required for the pumps.
In the cascade method higher TSBB's can be achieved but the average dry substance content is only insignificantly higher.
The use of lamellar separators, e.g. in the outlet of the activated sludge tank, has not proven itself because in this case, as in settling tanks, only gravity is at work and because the removal of the thickened sludge is problematic.
Another known possibility for concentrating activated sludge is offered by the membrane-activated sludge method. In it at least one filter membrane is provided in an activated sludge tank or in a separate filtration tank. In order to avoid clogging, the pores of this filter membrane should not exceed a maximal size of 0.4 μm, better 0.1 μm. It is furthermore necessary that a is sufficient flow be maintained on the filter membrane in order that the formation of a cover layer on the membrane is minimized. The water flowing off is free of undissolved substances due the pore size. Therefore, on the one hand good concentration of the activated sludge is possible. On the other hand, the dry substance content on the filter membrane should not exceed circa 12 g/l. Therefore, on the whole the solid content in the activated sludge tank is limited at the top. A high flow rate and in the case of a separate filtration tank a high recirculation rate must be maintained; this has the result that even unpurified wastewater passes to the filter membrane due to bypasses, which can lead to premature fouling. Furthermore, excess sludge must be separately removed.
WO 2004/002604 teaches a rake in which rotating sieve elements are provided in steps. Nozzles for cleaning the sieve elements are provided on an upper deflection roller. The rake is not suitable for concentrating activated sludge in an activated sludge tank. The use in an activated sludge tank is not mentioned.
DE 197 27 984 describes an apparatus for removing floating sludge from a wastewater purification tank in which a circulating dewatering belt extends to the surface of the water in the purification tank such that it slightly dips into the water. The material floating on the water surface is removed by the apparatus. This apparatus is not suitable for increasing the biological purification performance in an activated sludge tank.